NUCLEI OF PURE WATER. 13 



fact that whereas the relatively enormous fog particle evaporates at once 

 beyond the range of visibility, this process stops in case of certain of the 

 invisible particles making about 0.5 to 50 per cent of the total number 

 as the evaporation is more rapid in the manner specified. The remaining 

 fog particles evaporate completely. 



12. The same, continued. J. J. Thomson, Langevin and Bloch, and 

 others* have referred the persistence of pure water nuclei of about io~ 8 

 cm. in diameter, to the minimum of surface tension discovered by Reinold 

 and Riickerf for thicknesses of films of about the same value. Since all 

 fog particles are so much larger than this order of values, it is difficult to 

 see why, under quiet evaporation without any interference, they do not 

 all terminate in water nuclei, allowance being made for subsidence. Yet 

 under these circumstances the yield of water nuclei is least, being usually 

 within i per cent. Whatever losses may be due to coalescence should 

 be increased when the rate of evaporation is increased, because there is 

 more motion of the air relatively to the fog particles. Again, precisely 

 the reverse occurs, inasmuch as an increased rate of evaporation enor- 

 mously increases the yield of water nuclei. 



Moreover, the residual water nuclei may, on rapid evaporation, exceed 

 the order of io~ 6 cm. in diameter two or three times; or on slow evapor- 

 ation they may fall below 5Xio~ 7 cm. and yet persist for half an hour 

 or more. Under any circumstances they are graded. They appear to 

 diminish in size with extreme slowness in the lapse of time, so that an 

 appropriate interval of waiting will yield any size. 



13. The same, continued. Since the fog particles are absolutely pure 

 water (water condensed on water vapor), it is tempting to suggest electri- 

 cal charge as the cause of the observed persistence, such charge being 

 acquired either by friction during the motion of particles undergoing 

 rapid evaporation (influx of air) or by the mere act of evaporation. The 

 latter, like the minimum of surface tension, would require the same 

 persistence of all fog particles under conditions of quiet evaporation. As 

 has frequently been shown, this is not the case. A frictional mechanism, 

 suggested in view of the occurrence of convection during the period of 

 evaporation and influx of air, if in action, would account for the discrimi- 

 nation between fog particles as to survival. Thus drops of larger size are 

 stirred about for a longer time before complete evaporation, and they are 

 therefore more favorably circumstanced to persist, as they have been 

 found to do ; water nuclei should not be of the same size and they are not ; 

 etc. But all my experiments have failed to detect the amount of charge 

 commensurate with the persistence of nuclei. 



*J. J. Thomson: Conduction of Electricity through Gases, p. 152, 1903; C. T. R. 

 Wilson, Trans. St. Louis Electrical Congress, vol. i, pp. 364-378, 1904. 

 jReinold and Riicker: Proc. Roy. Soc., vol. 40, p. 441, 1886. 



